A radiological system essentially comprises an X-ray tube and a detector of such rays with an object to be examined such as the portion of the body of the patient being interposed therebetween. The detector (which may be constituted by a film-screen pair, for example), provides an image of the object after being exposed for an appropriate length of time and after the film has been developed. The quality of the image depends both on the characteristics of the object and on the parameters of the radiological system.
The radiological properties of an object are given by its thickness and by its composition, and these properties vary firstly from one patient to another and secondly from one part of the body to another. It is difficult to determine these characteristics accurately: in particular, it is difficult or even impossible to determine the composition of an object merely by means of a physical examination. The notion of equivalent thickness, as known in radiology, serves to reduce knowledge about these two variables to a problem in one dimension.
The equivalent thickness of an object is defined relative to a reference substance such as plexiglass or a substance simulating the absorption of an organ of given composition. Under accurate radiological conditions, i.e. with fixed configuration and exposure parameters, the equivalent thickness of an object placed in the radiation field is represented by the thickness of the reference substance that would provide the same quantity of energy at the detector, i.e. the same optical density when the detector is a film.
A doctor can make use of knowledge about the equivalent thickness of an object, for example, as a medical indication or for establishing statistics about patients.
The equivalent thickness of an object also depends on parameters of the radiological system. These parameters are generally classified in two categories:
"radiological" parameters such as the voltage V of the X-ray tube, the current I taken by the tube, the exposure time S, and the product I.times.S which defines the quantity of energy emitted; and
"configuration" parameters which are all of the parameters other than the radiological parameters that have an effect on the quality of the incident radiation on the detector, and not including the object.
For example, these configuration parameters may be:
a) the selected track of the rotary anode in the X-ray tube;
b) the selected size of the focus in the X-ray tube;
c) the selected filter interposed on the path of the beam of X-rays;
d) the selected magnification;
e) the selected distance between the focus and the image receiver;
f) the selected image receiver; and
g) the selected types of accessory present in the beam of X-rays, e.g. a compression pad, an anti-diffusion screen, etc.